Modulating molecular and nanoparticle transport in flexible polydimethylsiloxane membranes

The ability to fabricate flexible filtration membranes that can selectively separate particles of different sizes is of considerable interest. In this article, we describe a facile, reproducible and simple one-step method to produce pores in polydimethylsiloxane (PDMS) membranes. We embedded micron-sized NaHCO(3) particles in 50 micron thick PDMS films. After curing, the membranes were immersed in concentrated HCl acid. Pores were generated in the membrane by the evolution of CO(2) gas from the reaction of NaHCO(3) and HCl. High resolution Scanning Electron Microscope images clearly reveal the presence of openings on the surface and the cross-section of the membranes. Fluorescence and back-scattered electron imaging of porous PDMS membrane with embedded gold nanoparticles and comparison with non-porous PDMS membranes provided unambiguous evidence of pores in the membrane. Transport studies of molecular fluoresceinate ions, ions (sodium and chloride) and 240 nm polystyrene nanoparticles through these membranes demonstrate passable pores and existence of channels within the body of the membrane. Mechanically stretching the porous PDMS membrane and comparing the flow rates of fluoresceinate ions and the polystyrene beads through the stretched and unstretched membranes allowed a direct proof of the modulation of transport rate in the membranes. We show that stretching the membranes by 10% increases the flow rate of fluorescein molecules by 2.8 times and by a factor of approximately ~40% for the polystyrene nanoparticles.

Ultracompact beam splitters based on plasmonic nanoslits

An ultracompact plasmonic beam splitter is theoretically and numerically investigated. The splitter consists of a V-shaped nanoslit in metal films. Two groups of nanoscale metallic grooves inside the slit (A) and at the small slit opening (B) are investigated. We show that there are two energy channels guiding light out by the splitter: the optical and the plasmonic channels. Groove A is used to couple incident light into the plasmonic channel. Groove B functions as a plasmonic scatter. We demonstrate that the energy transfer through plasmonic path is dominant in the beam splitter. We find that more than four times the energy is transferred by the plasmonic channel using structures A and B. We show that the plasmonic waves scattered by B can be converted into light waves. These light waves redistribute the transmitted energy through interference with the field transmitted from the nanoslit. Therefore, different beam splitting effects are achieved by simply changing the interference conditions between the scattered waves and the transmitted waves. The impact of the width and height of groove B are also investigated. It is found that the plasmonic scattering of B is changed into light scattering with increase of the width and the height of B. These devices have potential applications in optical sampling, signal processing, and integrated optical circuits.

Investigating photoinduced charge transfer in carbon nanotube-perylene-quantum dot hybrid nanocomposites

In this study, we investigate photophysical and photoinduced current responses of a nanocomposite which consists of multiwalled carbon nanotubes (CNTs), thiol derivative perylene compound (ETPTCDI), and cadmium selenide quantum dots (QDs). These QDs as well as the ETPTCDI harvest photons and transfer their excited electrons or holes to CNTs to complete the circuit. Both QDs and ETPTCDI contribute charges to the carbon nanotubes, which increased the overall photon harvest efficiency of the nanocomposite. Herein, we investigate through a series of photophysical photoluminescence quenching studies the charge transfer between donors (QDs and ETPTCDI) and acceptor (CNTs). The incorporation of ETPTCDI into the nanocomposite significantly increases the adhesion between QDs and CNTs through bonding between QDs and thiol groups on ETPTCDI and π-π interactions between ETPTCDI and CNTs. Thus, ETPTCDI acted as a molecular linker between QDs and CNTs. Furthermore, a significant increase (>5 times) in the Stern-Volmer constant, K(sv), for QD emission after addition of ETPTCDI-tagged CNTs clearly indicates a large enhancement in the adhesion between CNTs and QDs. The nanocomposite shows a ∼2-4-fold increase in the photoconductivity when exposed to AM1.5 solar-simulated light. The damage to the nanocomposite from the intensity of the solar-simulated light is also investigated. The proposed nanocomposite has the potential for photovoltaic applications such as being the active component in a hybrid bulk heterojunction solar cell.

Photo-pens: a simple and versatile tool for maskless photolithography

We demonstrate conical pores etched in tracked glass chips for fabricating patterns at the micrometer scale. Highly fluorescent patterns based on photopolymerization of diacetylene films were formed by irradiating UV light through conical pores called "photo-pens". The properties of photopens were investigated through experiments, finite-difference-time-domain (FDTD) simulations and numerical calculations based on Fresnel equations. We show that the pattern dimensions are easily controlled by adjusting the exposure time. Thus, patterns with a range of dimensions can be fabricated without any need of changes in the pore diameter. Parallel patterning was also demonstrated by simultaneously exposing the films to photons through multiple pores in the chip. Our method provides an inexpensive, versatile, and efficient way for patterning without the use of sophisticated masks.

Carbon nanotube-MoS2 composites as solid lubricants

Solid lubricants (SLs) characterized by low coefficients of friction (mu) and wear rates (w) drastically improve the life span of instruments that undergo extreme frictional wear. However, the performance of SLs such as sputtered or nanoparticulate molybdenum disulfide (MoS(2)), tungsten disulfide (WS(2)), or graphite deteriorates heavily under extreme operational conditions such as elevated temperatures and high humidity. Here, we present our preliminary results, which demonstrate that composites of carbon nanotubes (CNTs) and MoS(2) produced by electrodeposition of MoS(2) on vertically aligned CNT films have low mu ( approximately 0.03) and w (approximately 10(-13) mm(3)/N.mm) even at 300 degrees C, which are about 2 orders of magnitude better than those of nanoparticulate MoS(2)-based coatings. The high load-bearing capacity of CNTs provides a strong enduring support to MoS(2) nanoclusters and is responsible for their ultralow w. The incorporation of these composites in liquid lubricants reduces the friction coefficient of the liquid lubricants by approximately 15%. The technique described here to produce SL coatings with extremely appealing frictional properties will provide valuable solutions for a variety of tribological applications where the coatings encounter high temperature, reduced pressure, and/or low- and high-humidity conditions.

Synthesis and characterization of quantum dot-polymer composites

In this study, we demonstrate a facile and simple synthesis of quantum dot (QD)-polymer composites. Highly fluorescent semiconducting CdSe/ZnS quantum dots were embedded in different commercially available polymers using one easy step. QD-polymer composite nanoparticles were also synthesized using template-assisted synthesis. In particular, we self-assembled lamellar micelles inside nanoporous alumina membranes which were used for the synthesis of mesoporous silica hollow nanotubes and solid nanorods. We observed that the addition of excess free octadecylamine (ODA) in the QD-silica solution resulted in gelation. The gelation time was found to be dependent on free ODA concentration. Similarly, the emission of QD-polymer composites was also found to be dependent on free ODA concentration. Highly purified QDs provided polymer composites that have a much lower emission compared to unpurified nanocomposites. This was attributed to passivation of the QD surfaces by amine, which reduced the surface defects and non-radiative pathways for excited QDs. Finally, highly fluorescent QD-polymer patterns were demonstrated on glass substrates which retained their emission in both polar and non-polar solvents.

Electric field-induced direct delivery of proteins by a nanofountain probe

We report nanofabrication of protein dot and line patterns using a nanofountain atomic force microscopy probe (NFP). Biomolecules are continuously fed in solution through an integrated microfluidic system, and deposited directly onto a substrate. Deposition is controlled by application of an electric potential of appropriate sign and magnitude between the probe reservoir and substrate. Submicron dot and line molecular patterns were generated with resolution that depended on the magnitude of the applied voltage, dwell time, and writing speed. By using an energetic argument and a Kelvin condensation model, the quasi-equilibrium liquid-air interface at the probe tip was determined. The analysis revealed the origin of the need for electric fields in achieving protein transport to the substrate and confirmed experimental observations suggesting that pattern resolution is controlled by tip sharpness and not overall probe aperture. As such, the NFP combines the high-resolution of dip-pen nanolithography with the efficient continuous liquid feeding of micropipettes while allowing scalability to 1- and 2D probe arrays for high throughput.

Synthesis and characterization of polydiacetylene films and nanotubes

We report here the synthesis and characterization of polydiacetylene (PDA) films and nanotubes using layer-by-layer (LBL) chemistry. 10,12-Docosadiyndioic acid (DCDA) monomer was self-assembled on flat surfaces and inside of nanoporous alumina templates. UV irradiation of DCDA provided polymerized-DCDA (PDCDA) films and nanotubes. We have used zirconium-carboxylate interlayer chemistry to synthesize PDCDA multilayers on flat surfaces and in nanoporous template. PDCDA multilayers were characterized using optical (UV-vis, fluorescence, ellipsometry, FTIR) spectroscopies, ionic current-voltage ( I- V) analysis, and scanning electron microscopy. Ellipsometry, FTIR, electronic absorption and emission spectroscopies showed a uniform DCDA deposition at each deposition cycle. Our optical spectroscopic analysis indicates that carboxylate-zirconium interlinking chemistry is robust. To explain the disorganization in the alkyl portion of PDCDA multilayer films, we propose carboxylate-zirconium interlinkages act as "locks" in between PDCDA layers which restrict the movement of alkyl portion in the films. Because of this locking, the induced-stresses in the polymer chains can not be efficiently relieved. Our ionic resistance data from I- V analysis correlate well with calculated resistance at smaller number of PDCDA layers but significantly deviated for thicker PDCDA nanotubes. These differences were attributed to ion-blocking because some of the PDCDA nanotubes were totally closed and the nonohmic and permselective ionic behaviors when the diameter of the pores approaches the double-layer thickness of the solution inside of the nanotubes.

Fluorescence resonance energy transfer in polydiacetylene liposomes

Conjugated polydiacetylene (PDA) possessing stimuli-responsive properties has been intensively investigated for developing efficient sensors. We report here fluorescence resonance energy transfer (FRET) in liposomes synthesized using different molar ratios of dansyl-tagged diacetylene and diacetylene-carboxylic acid monomers. Photopolymerization of diacetylene resulted in cross-linked PDA liposomes. We used steady-state electronic absorption, emission, and fluorescence anisotropy (FA) analysis to characterize the thermal-induced FRET between dansyl fluorophores (donor) and PDA (acceptor). We found that the monomer ratio of acceptor to donor ( R ad) and length of linkers (functional part that connects dansyl fluorophores to the diacetylene group in the monomer) strongly affected FRET. For R ad = 10 000, the acceptor emission intensity was amplified by more than 18 times when the liposome solution was heated from 298 to 338 K. A decrease in R ad resulted in diminished acceptor emission amplification. This was primarily attributed to lower FRET efficiency between donors and acceptors and a higher background signal. We also found that the FRET amplification of PDA emissions after heating the solution was much higher when dansyl was linked to diacetylene through longer and flexible linkers than through shorter linkers. We attributed this to insertion of dansyl in the bilayer of the liposomes, which led to an increased dansyl quantum yield and a higher interaction of multiple acceptors with limited available donors. This was not the case for shorter and more rigid linkers where PDA amplification was much smaller. The present studies aim at enhancing our understanding of FRET between fluorophores and PDA-based conjugated liposomes. Furthermore, receptor tagged onto PDA liposomes can interact with ligands present on proteins, enzymes, and cells, which will produce emission sensing signal. Therefore, using the present approach, there exist opportunities for designing FRET-based highly sensitive and selective chemical and biochemical sensors.

Mechanical, tribological, and biocompatibility properties of ZrN-Ag nanocomposite films

Nanocomposite films of ZrN-Ag were produced by reactive unbalanced magnetron sputtering, and their structural, chemical, mechanical, tribological, haemocompatibility, and antibacterial properties were studied as a function of film composition. The films formed a dense and homogeneous microstructure whereby nanocrystals of Ag are distributed evenly throughout the ZrN matrix. For small additions of silver, the hardness was found to increase, whereas the elastic modulus was found to decrease drastically. In the process of optimizing the deposition conditions, three kinds of coatings were prepared on 316 L surgical steel and tested by accelerated electrochemical polarization tests in order to detect the influence of Ag and the value of the bias potential on the corrosion performance of the system. Films produced under the optimum deposition conditions were, subsequently, deposited on medical grade Ti-Al-V and worn against ball-bearing steel using a ball-on-disk tribotester in bovine serum and were found to have superior tribological properties compared with single-phase ZrN coatings. The haemocompatibility of the films was assessed by investigating the adsorption of human serum albumin and fibrinogen on samples with different phase compositions. Quantification of the protein adsorption was carried out using spectroscopic ellipsometry, which confirmed the haemocompatibility of the films. Antibiotic activity of the films was quantified by incubating the films in bacterial cultures, namely, Staphylococcus epidermis, Staphylococcus aureus, and Escherichia coli. Films with a silver content > 10% exhibited superior antibacterial activity compared with the uncoated samples.

Modulating fluorescence resonance energy transfer in conjugated liposomes

We report here a novel system where the rate of energy transfer is based on changes in the spectral overlap between the emission of the donor and the absorption of the acceptor (J) as well as changes in the quantum yield of the acceptor. We use the fluorophore dansyl as the donor and polydiacetylene (PDA) as the acceptor to demonstrate the modulation of FRET through conformationally induced changes in the PDA absorption spectrum following thermal treatment that converts the PDA backbone of the liposome from the blue form to the red form. Energy transfer was found to be significantly more efficient from dansyl to the red-form PDA. These findings support the basis of a new sensing platform that utilizes J-modulated FRET as an actuating mechanism.

Transplantation of hNT neurons into the ischemic cortex: Cell survival and effect on sensorimotor behavior

Cell transplantation offers a potential new treatment for stroke. Animal studies using models that produce ischemic damage in both the striatum and the frontal cortex have shown beneficial effects when hNT cells (postmitotic immature neurons) were transplanted into the ischemic striatum. In this study, we investigated the effect of hNT cells in a model of stroke in which the striatum remains intact and damage is restricted to the cortex. hNT cells were transplanted into the ischemic cortex 1 week after stroke induced by distal middle cerebral artery occlusion (dMCAo). The cells exhibited robust survival at 4 weeks posttransplant even at the lesion border. hNT cells did not migrate, but they did extend long neurites into the surrounding parenchyma mainly through the white matter. Neurite extension was predominantly toward the lesion in ischemic animals but was bidirectional in uninjured animals. Extension of neurites through the cortex toward the lesion was also seen when there was some surviving cortical tissue between the graft and the infarct. Prolonged deficits were obtained in four tests of sensory-motor function. hNT-transplanted animals showed a significant improvement in functional recovery on one motor test, but there was no effect on the other three tests relative to control animals. Thus, despite clear evidence of graft survival and neurite extension, the functional benefit of hNT cells after ischemia is not guaranteed. Functional benefit could depend on other variables, such as infarct location, whether the cells mature, the behavioral tests employed, rehabilitation training, or as yet unidentified factors.

Protein biosensors based on biofunctionalized conical gold nanotubes

There is increasing interest in the concept of using nanopores as the sensing elements in biosensors. The nanopore most often used is the alpha-hemolysin protein channel, and the sensor consists of a single channel embedded within a lipid bilayer membrane. An ionic current is passed through the channel, and analyte species are detected as transient blocks in this current associated with translocation of the analyte through the channel-stochastic sensing. While this is an extremely promising sensing paradigm, it would be advantageous to eliminate the very fragile lipid bilayer membrane and perhaps to replace the biological nanopore with an abiotic equivalent. We describe here a new family of protein biosensors that are based on conically shaped gold nanotubes embedded within a mechanical and chemically robust polymeric membrane. While these sensors also function by passing an ion current through the nanotube, the sensing paradigm is different from the previous devices in that a transient change in the current is not observed. Instead, the protein analyte binds to a biochemical molecular-recognition agent at the mouth of the conical nanotube, resulting in complete blockage of the ion current. Three different molecular-recognition agents, and correspondingly three different protein analytes, were investigated: (i) biotin/streptavidin, (ii) protein-G/immunoglobulin, and (iii) an antibody to the protein ricin with ricin as the analyte.

Smart Nanotubes for Biotechnology

Nanotechnology concerns the science of very small particles and deals with both the fundamental aspects of understanding the properties of such nanoparticles and with developing technological applications of nanoparticles. Biomedical and biotechnological applications of nanoparticles have been of special recent research and development interest, with potential applications that include use of nanoparticles as drug (or DNA) delivery vehicles, and as components in medical diagnostic kits, biosensors and membranes for bioseparations. Spherical nanoparticles are typically used for such applications, but this only reflects the fact that spheres are easier to make than nanoparticles having other shapes. Micro and nanotubes - structures that resemble tiny drinking straws - are alternatives and may offer advantages over spherical nanoparticles for some applications. This article discusses different approaches for making micro and nanotubes and reviews the current status of efforts to develop biomedical and biotechnological applications of these tubular structures.

DNA−Nanotube Artificial Ion Channels

There is considerable interest in developing chemical devices that mimic the function of biological ion channels. We recently described such a device, which consisted of a single conically shaped gold nanotube embedded within a polymeric membrane. This device mimicked one of the key functions of voltage-gated ion channels: the ability to strongly rectify the ionic current flowing through it. The data obtained were interpreted using a simple electrostatic model. While the details are still being debated, it is clear that ion-current-rectification in biological ion channels is more complicated and involves physical movement of an ionically charged portion of the channel in response to a change in the transmembrane potential. We report here artificial ion channels that rectify the ion current flowing through them via this "electromechanical" mechanism. These artificial channels are also based on conical gold nanotubes, but with the critical electromechanical response provided by single-stranded DNA molecules attached to the nanotube walls.

Conical-Nanotube Ion-Current Rectifiers: The Role of Surface Charge

Ion channels are protein pores that span cell membranes and open and close in response to stimuli like changes in the transmembrane potential, binding of a ligand, or mechanical stress. When open, ions pass through the pore, and hence across the cell membrane, and when closed, ion-transport is precluded. Hence, these channels are nanodevices that have a current-rectification function. There is intense research effort aimed at understanding the molecular-level mechanism for this function. One approach for elucidating the mechanism is to construct a simple abiotic system that mimics this function and to use the mechanistic details of this mimic as a guide to understand the more complex biological channel. We describe here such an abiotic mimic: a synthetic membrane that contains a single conical gold nanotube. The advantage of this mimic is that the surface charge and chemistry of the nanotube wall can be varied, at will, by judicious choice of electrolyte or by thiol chemisorption. This has allowed us to make conical Au nanotubes that rectify the ion current and, just as importantly, to definitively elucidate the mechanism of this function.

DNA-Functionalized Nanotube Membranes with Single-Base Mismatch Selectivity

We describe synthetic membranes in which the molecular recognition chemistry used to accomplish selective permeation is DNA hybridization. These membranes contain template-synthesized gold nanotubes with inside diameters of 12 nanometers, and a "transporter" DNA-hairpin molecule is attached to the inside walls of these nanotubes. These DNA-functionalized nanotube membranes selectively recognize and transport the DNA strand that is complementary to the transporter strand, relative to DNA strands that are not complementary to the transporter. Under optimal conditions, single-base mismatch transport selectivity can be obtained.

Template synthesis of gold nanotubes in an anodic alumina membrane

Nanotube-containing membranes prepared by the template method show promise for use as highly selective filters for membrane-based chemical and biological separations. Most of the work to date has been done on gold nanotubes prepared by electroless deposition of Au within the pores of polymeric filtration membranes. These polymeric filters have very low porosities (< 1%), and, as a result, the flux through Au nanotube membranes based on these templates is very low. In contrast, the other popular template membranes-anodic aluminas-have high porosities-30% to 50%. In spite of this potential advantage of anodic alumina templates, there have been no reports of electrolessly plated Au nanotubes within the pores of these templates. This is because the electroless plating method used to deposit Au nanotubes in polymeric templates does not work in aluminas. We have developed a modified electroless plating strategy that can be used to deposit high-quality Au nanotubes within the pores of the alumina templates. We describe this new plating method here.

Smart nanotubes for biomedical and biotechnological applications

Nanoparticles are being developed for a host of biomedical and biotechnological applications including drug delivery, enzyme immobilization and DNA transfection. Spherical nanoparticles are typically used for such applications, but this only reflects the fact that spheres are easier to make than other shapes. Micro- and nanotubes--structures that resemble tiny drinking straws--are alternatives and may offer advantages over spherical nanoparticles for some applications. This article discusses four different approaches to making micro- and nanotubes and reviews the current status of efforts to develop biomedical and biotechnological applications of these tubular structures.

Layer-by-Layer Nanotube Template Synthesis

Electroless deposition of gold on the pore walls of polycarbonate templates is currently the best known method for controlling inside diameters of template-synthesized nanotubes. It would be very useful to have alternative template-based synthetic chemistries that yield nanotubes composed of other materials, but which still allow for precise control over the nanotube wall thickness and i.d. A film-formation process that is based on layer-by-layer deposition of the film-forming material along the pore walls of the template membrane provides this desired alternative synthetic chemistry. We describe here the use of Mallouk's alpha,omega-diorganophosphonate/Zr layer-by-layer film-forming method for preparing nanotubes within the pores of alumina template membranes. We have found that this method allows accurate, quantitative, and predictable control over the wall thickness, and thus i.d., of the layered nanotubes obtained.

The emerging field of nanotube biotechnology

Nanoparticles are being developed for a host of biomedical and biotechnological applications, including drug delivery, enzyme immobilization and DNA transfection. Spherical nanoparticles are typically used for such applications, which reflects the fact that spheres are easier to make than other shapes. Micro- and nanotubes--structures that resemble tiny drinking straws--are alternatives that might offer advantages over spherical nanoparticles for some applications. This article discusses four approaches for making micro- and nanotubes, and reviews the current status of efforts to develop biomedical and biotechnological applications of these tubular structures.